(1) Field of the Invention
The present invention relates to shoes and a process for producing the same, more specifically to shoes having a sole of a two-layer structure such as safety shoes and sport shoes and a process for producing the same.
(2) Description of the Related Art
In general, safety shoes are divided, depending on a difference in sole materials, broadly into synthetic rubber sole safety shoes using synthetic rubber for the material and foamed polyurethane sole safety shoes using foamed polyurethane for the material.
The synthetic rubber sole safety shoes have the advantages that they do not have a hydrolytic property and that they have a high physical strength and are excellent in an oil resistance, a chemical resistance and a heat resistance, and therefore they are suitable to be used in the fields of the iron and steel industry, the mining industry, the construction industry, the chemical handling industry, and the like, in which heat, chemicals (acids, alkalis, and solvents) and oils are handled in large quantities, and working conditions are relatively severe. However, the synthetic rubber sole safety shoes are inferior in lightweight and a cushioning property (shock absorbing property) which are the most important in terms of an easiness in wearing, and have the defect that they are liable to bring about fatigue after wearing them over a long period of time.
In contrast with this, the foamed polyurethane sole safety shoes have lightweight and a cushioning property (shock absorbing property) which are the most important in terms of an easiness in wearing, and have the advantage that they cause little fatigue even after wearing them over a long period of time. However, they have the defects that they have a hydrolytic property and a low physical strength and are inferior in an oil resistance and a chemical resistance and that they have a low heat resistance and are liable to be scratched.
In recent years, an easiness in wearing tends to be regarded as being important also in requirements to safety shoes, and demand to the foamed polyurethane sole safety shoes is increasing. Under such circumstances, various improvements have been made in order to modify the preceding defects of the foamed polyurethane sole safety shoe.
Known as such shoes are, for example, foamed polyurethane two-layer structure sole safety shoes (Japanese Utility Model Publication No. Hei 1-44084) in which a large part of a heel is composed of polyurethane having a high foaming rate and a soft cushioning property, and a circumference thereof is covered with polyurethane having a low foaming rate, and a lower circumference of a vamp is covered with a thin sheet formed out of polyurethane having a high foaming rate, so that the heel is provided with a high cushioning property, and the effect of preventing the vamp from peeling off from the polyurethane sole can be provided.
Further, known as well are two-layer safety shoes (Japanese Utility Model Publication No. Hei 6-36725) in which a rubber material having a high abrasion resistance and a high oil resistance is used in part of a sole, and the other part of the sole is formed out of a flexible polyurethane material.
However, since the foamed polyurethane two-layer structure sole safety shoes disclosed in Japanese Utility Model Publication No. Hei 1-44084 described above is just made of foamed polyurethane, they have the problems that temperatures in the inside of the shoes are elevated during use due to a heat accumulating property which is a characteristic of polyurethane, and therefore that the inside of the shoes is liable to be hot and humid, and the shoes are liable to be scratched. Also, the foamed polyurethane two-layer structure sole safety shoes are susceptible to hydrolysis as compared with the synthetic rubber safety shoes and are inferior thereto in an oil resistance, a heat resistance and a chemical resistance, and therefore they still have a problem in terms of less durability. Further, they have a problem that use at a place where working conditions are relatively severe is restricted.
Since the safety shoes described in Japanese Utility Model Publication No. Hei 6-36725 employ foamed polyurethane as a flexible material, they have the same problems as those described above, that is, the problems that the shoes are liable to be hot and humid in the inside thereof and are liable to be scratched and that they are inferior to the rubber ones in durability and are restricted in use. Further, since the shoes are produced after forming a grounding plate with projections and a thin plate with windows in order to form the sole, they have many number of parts and dead parts and have many problems in terms of workability and cost.
Accordingly, synthetic rubber and foamed polyurethane safety shoes have the advantages and disadvantages as described above at present, and therefore the existing state is that the optimum safety shoes are suitably selected in light of the characteristics of both and use conditions. Desired are safety shoes endowed with the characteristics of both all together, that is, safety shoes which have no hydrolytic property, a high physical strength, an excellent oil resistance, chemical resistance and heat resistance, lightweight, and a cushioning property (shock absorbing property), and are less liable to bring about fatigue even after wearing them over a long period of time and which are less liable to be hot and humid in the inside thereof and to be scratched and have a good working efficiency as well as are capable of being produced at a low cost.
On the other hand, rubber sheet materials for a sole prepared by putting a sponge blend composition and a solid blend composition together to integrally mold them at the same time have recently been known (Japanese Patent Publication No. Sho 35-13127 and Japanese Patent Application Laid-Open No. Hei 6-155646).
The materials disclosed in Japanese Patent Publication No. Sho 35-13127 and Japanese Patent Application Laid-Open No. Hei 6-155646 have something partially in common with the present invention, but the above materials are the rubber sheet materials for a sole. Accordingly, in order to produce footgears having sheet-shaped soles and the like from these materials, there has to be employed a troublesome method in which sheet-shaped soles are first prepared, and then the above sole and a vamp are put together by adhesion, sewing means or the like, and since the above method is not directed to producing footgears having sheet-shaped soles by integrally molding, it has the problems that the working efficiency in the production steps is poor, leading to an increase in the cost.
As apparent from descriptions in these specifications, only sheet-shaped products having as small thickness as 0.5 to 5 mm can be produced in a sponge part and a solid part due to a difference in an expansion coefficient, influence exerted by foam gas, and a difference in vulcanizing time between the sponge part and the solid part. Products having a larger thickness than the range described above are liable to cause warp and detachment of the two layers due to a difference in an expansion coefficient, and influence exerted by foam gas and the like as described above, and therefore they cannot be applied to shoes having thick soles provided with heels, and safety shoes. In addition, there are many problems that soles having no specified physical properties are produced due to a difference in vulcanizing time between a sponge part and a solid part, and therefore insufficiently qualified sole products are produced.